/Engine/Source/Runtime/Engine/Classes/Animation/AnimData/IAnimationDataController.h
Sets the total play-able length in seconds. Broadcasts a EAnimDataModelNotifyType::SequenceLengthChanged notify if successful. The number of frames and keys for the provided length is recalculated according to the current value of UAnimDataModel::FrameRate.
void SetNumberOfFrames ( [FFrameNumber](API\Runtime\Core\Misc\FFrameNumber) NewLengthInFrames, bool bShouldTransact )